This invention relates to a rare earth oxide, a basic rare earth carbonate, methods for preparing them, and a phosphor and ceramic obtained therefrom.
Among rare earth oxides, a spherical rare earth oxide having an average particle diameter of 0.2 to 1 xcexcm as measured by a Fisher sub-sieve sizer (this average particle diameter is sometimes referred to as Fisher diameter, hereinafter) is obtainable by a known method as disclosed in Wataya et al., U.S. Pat. No. 5,879,647 (JP-A 10-139427). Also, a method for preparing finer spherical particles having an average particle diameter of 0.1 to 0.3 xcexcm is known from JP-A 10-139427. A method for preparing spherical particles having an average particle diameter of 2 to 6 xcexcm is disclosed, for example, in JP-A 8-59233.
These methods, however, are difficult to produce spherical rare earth oxide particles having a Fisher diameter from more than 0.5 xcexcm to less than 2 xcexcm.
Yttrium-europium oxide phosphors and yttrium-gadolinium-europium oxide phosphors are used in plasma display and medical diagnostic x-ray systems as the red phosphor. The plasma display is promising as a large-size flat display panel. Yttrium-europium oxide phosphors and yttrium-gadolinium-europium oxide phosphors are attractive as plasma display red phosphors having a high luminous efficiency to excitation light of 147 nm emitted by xenon plasma.
While yttrium-gadolinium-europium borate phosphors are also known as the red phosphor for plasma displays, the yttrium-europium oxide phosphors and yttrium-gadolinium-europium oxide phosphors are potential candidates since they are superior in color purity and lifetime despite a lower luminous efficiency.
For medical diagnostic x-ray systems, yttrium-gadolinium-europium oxide phosphors are regarded promising because of high luminous efficiency to x-rays.
For such display panels as plasma display panels (PDP), to increase their brightness is an important task in improving display performance.
The brightness of panels can be increased, for example, by increasing the brightness of phosphor itself. It is believed that the brightness of panels is largely affected by the coating property of phosphor to cells. With respect to the coating property of phosphor, those phosphors which can be applied to plasma display cells in a uniform, least irregular state are regarded preferable. With respect to the shape of phosphor, particles of small size, equal diameter and identical shape are regarded preferable because uniform coating property is improved.
The particle size and shape of phosphor, especially the particle size of phosphor, depend on the particle size of a raw material. In general, using a raw material having a less variation of particle diameter or a sharper particle size distribution, a phosphor having a sharper particle size distribution is obtained. The raw material powder is thus required to have a sharper particle size distribution.
However, a microscopic observation of conventional raw material oxide revealed that even a raw material powder having a sharper particle size distribution contained particles of differing size. Such a raw material powder was regarded to have a sharper particle size distribution for the mere reason that the difference in particle size was relatively small or particles had somewhat similar shapes. The phosphor prepared from such a raw material powder contains particles of differing size.
An object of the invention is to provide a spherical rare earth oxide and spherical basic rare earth carbonate having an average particle diameter from more than 0.5 xcexcm to less than 2 xcexcm which is difficult to obtain in the prior art, methods for preparing them, a phosphor and ceramic obtained therefrom.
Another object of the invention is to provide a phosphor having a uniform particle diameter and a sharp particle size distribution, suitable for typical use as a red phosphor in displays and in medical diagnostic x-ray systems, and a method for preparing the same.
From a study on precipitating conditions including controlled concentrations of rare earth ions, carbonic acid or carbonate ions, and ammonia or ammonium ions in aqueous solution, we have reached the present invention. More specifically, when rare earth oxides are used as a raw material to form ceramics and phosphors, the characteristics of products are largely affected by the particle shape, particle size and particle size distribution of rare earth oxides. The rare earth oxide particles have a variety of shapes including irregular, tabular, angular, and spherical shapes. Of these, spherical particles are one of the particle shapes regarded most preferable as the raw material. Among such spherical particles of rare earth oxide, a fraction of particles having a diameter from more than 0.5 xcexcm to less than 2 xcexcm has never been available in the art, and we tried to obtain this fraction of particles. We reached the basic concept of synthesis that better results are obtainable by generating a suitable amount of particle nuclei serving as crystal seeds in a liquid phase and thereafter, controlling the concentration of a precipitant in the liquid phase such that the initially generated particle nuclei may be grown without generating new particle nuclei. Since amorphous particles were believed preferable to obtain spherical particles, a choice was made of basic rare earth carbonates from which amorphous rare earth salts were readily obtainable. It was found that an effective means for obtaining a basic carbonate was to homogeneously add ammonia or ammonium ions and carbonic acid or carbonate ions to a rare earth ion-containing liquid phase.
The addition of a precipitant is readily accomplished by adding urea to the solution and heating the solution at a temperature of 80xc2x0 C. or higher.
However, the method of obtaining a precipitate of basic rare earth carbonate by adding urea to a solution of rare earth salt and heating the solution is difficult to produce particles having a Fisher diameter in excess of 1 xcexcm because an excessive amount of precipitate forms in the solution if the concentration of urea in the solution is too high.
Paying attention to the change of the urea concentration in the solution, we have found that the number of particles generated in the solution at an initial stage of reaction can be reduced by adjusting the urea concentration so as not to become high, and that the initially generated particles can be grown larger by suppressing further particle generation after the initial stage of reaction.
Moreover, using a spherical rare earth oxide of uniform particle diameter and uniform particle shape having an average particle diameter of 0.5 to 2 xcexcm which is obtained by the above method, we tried to produce a consistent yttrium-europium oxide phosphor or yttrium-gadolinium-europium oxide phosphor having a uniform particle diameter of 0.5 to 2 xcexcm.
Nowadays, it becomes customary in the medical field to store digital radiographic data for diagnosis. To increase the resolution of diagnostic images, the phosphor is required to have a high luminous efficiency to x-rays, a fine particle size, luminous characteristics having a high sensitivity to the detector, and good coating property. We thus tried to produce a consistent yttrium-gadolinium-europium oxide phosphor having a uniform particle diameter of 0.5 to 2 xcexcm as the phosphor capable of meeting these requirements.
We have found that a yttrium-europium or yttrium-gadolinium-europium oxide phosphor having a uniform particle diameter can be produced by heating a consistent, spherical, coprecipitated yttrium-europium or yttrium-gadolinium-europium oxide having a particle diameter of 0.5 to 2 xcexcm, obtained as above, at a temperature of 1,100 to 1,800xc2x0 C. Advantageously, the oxide used as the raw material has a so small particle diameter of 0.5 to 2 xcexcm that crystal growth is facilitated. The oxide particles used as the raw material have a uniform size so that equal crystal growth occurs among individual particles during heat treatment at the same temperature. A phosphor can be produced by causing the starting oxide to undergo crystal growth without a need for admitting a flux which is commonly used in phosphor production such as boric acid, barium chloride or ammonium chloride. This leads to the advantage of simplifying the phosphor producing process. The phosphor thus obtained is useful as a red phosphor in plasma display systems and as a phosphor in medical diagnostic x-ray systems.
The above and other objects are attained by the present invention which is defined below.
(1) A rare earth oxide in the form of substantially spherical particles having an average particle diameter Df of 0.5 xcexcm less than Df less than 2.0 xcexcm as measured by a Fisher sub-sieve sizer and a particle diameter De of 0.5 xcexcm less than De less than 2.0 xcexcm as observed under an electron microscope.
(2) The rare earth oxide of (1) which contains at least 60 mol % of an oxide of at least one element selected from the group consisting of Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and less than 40 mol % of an oxide of at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, and Eu.
(3) A basic rare earth carbonate in the form of substantially spherical amorphous particles having an average particle diameter Df of 1.0 xcexcm less than Df less than 3.0 xcexcm as measured by a Fisher sub-sieve sizer and a particle diameter De of 1.0 xcexcm less than De less than 3.0 xcexcm as observed under an electron microscope.
(4) The basic rare earth carbonate of (3) which contains at least 60 mol % of a basic carbonate of at least one element selected from the group consisting of Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and less than 40 mol % of a basic carbonate of at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, and Eu.
(5) A method for preparing the rare earth oxide of (1) or (2), comprising firing the basic rare earth carbonate of (3).
(6) A method for preparing the rare earth oxide of (1) or (2), comprising the steps of heating and ripening an aqueous solution of water-soluble rare earth salts at a temperature of at least 80xc2x0 C. while adding urea to the aqueous solution so as to keep the concentration of urea at a substantially constant level of up to 50 g/liter, thereby forming a basic rare earth carbonate; and firing the basic rare earth carbonate.
(7) The method of (5) or (6) wherein the firing temperature is at least 600xc2x0 C.
(8) A method for preparing the basic rare earth carbonate of (3), comprising the steps of adding urea to an aqueous solution of water-soluble rare earth salts so as to keep the concentration of urea at a substantially constant level of up to 50 g/liter; and then heating and ripening the aqueous solution at a temperature of at least 80xc2x0 C.
(9) The method of (8) further comprising the step of preparing said aqueous solution of water-soluble rare earth salts using deionized water having a silicon content of up to 0.5 ppm.
(10) A phosphor obtained from a rare earth oxide in the form of substantially spherical particles having an average particle diameter Df of 0.5 xcexcm less than Df less than 2.0 xcexcm as measured by a Fisher sub-sieve sizer and a particle diameter De of 0.5 xcexcm less than De less than 2.0 xcexcm as observed under an electron microscope.
(11) The phosphor of (10) which contains at least 60 mol % of an oxide of at least one element selected from the group consisting of Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and less than 40 mol % of an oxide of at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, and Eu.
(12) A consistent yttrium-europium oxide phosphor having a uniform particle diameter of 0.5 xcexcm to 2 xcexcm.
(13) The yttrium-europium oxide phosphor of (12) wherein the content of boron and barium combined is not greater than 20 ppm.
(14) A method for preparing a yttrium-europium oxide phosphor, comprising the step of heating a consistent, spherical, monodisperse, coprecipitated yttrium-europium oxide having a uniform particle diameter of 0.5 xcexcm to 2 xcexcm at a temperature between 1,100xc2x0 C. and 1,800xc2x0 C., thereby yielding a yttrium-europium oxide phosphor having the same particle diameter as the coprecipitated oxide.
(15) A consistent yttrium-gadolinium-europium oxide phosphor having a uniform particle diameter of 0.5 xcexcm to 2 xcexcm.
(16) The yttrium-gadolinium-europium oxide phosphor of (15) wherein the content of boron and barium combined is not greater than 20 ppm.
(17) A method for preparing a yttrium-gadolinium-europium oxide phosphor, comprising the step of heating a consistent, spherical, coprecipitated yttrium-gadolinium-europium oxide having a uniform particle diameter of 0.5 xcexcm to 2 xcexcm at a temperature between 1,100xc2x0 C. and 1,800xc2x0 C., thereby yielding a yttrium-gadolinium-europium oxide phosphor having the same average particle diameter as the coprecipitated oxide.
(18) A ceramic obtained from a rare earth oxide in the form of substantially spherical particles having an average particle diameter Df of 0.5 xcexcm less than Df less than 2.0 xcexcm as measured by a Fisher sub-sieve sizer and a particle diameter De of 0.5 xcexcm less than De less than 2.0 xcexcm as observed under an electron microscope.
(19) The ceramic of (18) which contains at least 60 mol % of an oxide of at least one element selected from the group consisting of Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and less than 40 mol % of an oxide of at least one element selected from the group consisting of La, Ce, Pr, Nd, Sm, and Eu.